The invention relates to an optical writing and/or reading device comprising a radiation source for generating a radiation beam, an optical system for directing the radiation beam onto a radiation-sensitive detector via a record carrier, the radiation beam producing a radiation spot on the record carrier, means for scanning a servotrack on the record carrier by the radiation spot in a longitudinal direction, the detector comprising means for deriving a first and a second detection signal which are indicative of the power in a first and a second part, respectively, within the cross-section of the radiation beam received by the detector, said two parts being located at both sides of an imaginary line whose direction corresponds to the track direction, signal combination means for deriving a difference signal which is indicative of the difference between the two detection signals, said device further comprising an information-regaining circuit for regaining information from the difference signal.
A device of this type is known from EP-A-0265984, to which U.S. Pat. No. 5,023,856 corresponds. The device described in this Application is a device for recording and/or reading information in/from a servotrack having a periodical radial track wobble. The frequency of the wobble is modulated in accordance with a longitudinal position-information signal. As a result of this track wobble the difference signal has an FM modulation which is free from DC and which represents the position-information signal. This position-information signal is regained by means of a signal-regaining circuit in the form of an FM demodulator. In this way position information can be regained from the push-pull signal when the recorded information is being read, which is desirable for an efficient control. Also during recording information, in which the intensity of the radiation beam is modulated, information about the position of the scanned potion of the servotrack can be derived. For a reliable regaining of the position-information signal it is desirable that the wobble/noise ratio in the difference signal is sufficiently high. It has been found that a small deviation in the positioning of the detector already leads to a considerable deterioration of the wobble/noise ratio.